Mineral insulated cables at a panel
board
Mineral-insulated copper-clad cable is a variety
of
electrical cable made from
copper conductors inside a copper sheath, insulated by inorganic
magnesium oxide powder. The name is
often abbreviated to MICC or MI cable, and colloquially known as
pyro (because one vendor for this product is a
company called
Pyrotenax). A
similar product sheathed with metals other than copper is called
mineral insulated metal sheathed (MIMS) cable. MI cable
fulfills the
passive fire
protection called
circuit
integrity, which is intended to provide operability of critical
electrical circuits during a fire. It is subject to strict
listing and approval use
and compliance.
MI cable is made by placing copper rods inside a circular copper
tube and filling the intervening spaces with dry magnesium oxide
powder. The overall assembly is then pressed between rollers to
reduce its diameter (and increase its length). Up to seven
conductors are often found in an MI cable, with up to 19 available
from some manufacturers.
Since MI cables use no organic material as insulation (except at
the ends), they are more resistant to fires than structures that
are wired with plastic-insulated cables. MI cables are used in
critical
fire protection
applications such as alarm circuits, fire pumps, and smoke control
systems. In process industries handling flammable fluids MI cable
is used where small fires would otherwise cause damage to control
or power cables. MI cable is also highly resistant to ionising
radiation and so finds applications in
instrumentation for
nuclear
reactors and nuclear physics apparatus.
The metal tube surrounding the conductors effectively shields
circuits in MI cable from electromagnetic interference. The metal
sheath provides protection against accidental contact with
energised circuit conductors.
MI cables may be covered with a plastic sheath, coloured for
identification purposes. The plastic sheath also provides
additional corrosion protection for the copper sheath.
History
The first patent for MI cable was issued to the Swiss inventor
Arnold Francois Borel in 1896. Initially the insulating mineral was
described in the patent application as pulverized glass, silicious
stones, or
asbestos, in powdered form. Much
development ensued by the French company Societe Alsacienne de
Construction Mechaniques.
Commercial production began in 1932 and much
mineral-insulated cable was used on ships such as the Normandie and oil
tankers, and in such critical applications as the Louvre
museum. In 1937 a British company
Pyrotenax, having
purchased patent rights to the product from the French company,
began production. During the
Second
World War much of the company's product was used in military
equipment.
About 1947 the British Cable Maker's Association investigated the
option of manufacturing a mineral-insulated cable that would
compete with the Pyrotenax product. The manufacturers of the
products "Bicalmin" and "Glomin" eventually merged with the
Pyrotenax company.
The Pyrotenax company introduced an aluminum sheathed version of
its product in 1964. MI cable is now manufactured in several
countries. Pyrotenax is now a wholly owned subsidiary of
Tyco Corporation.
Purpose and use
MI cables are used to provide
circuit
integrity for power and control circuits of critical equipment,
such as the following examples:
Heating cable
A similar appearing product is mineral-insulated
trace heating cable, in which the conductors
are made of a high-resistance alloy. A heating cable is used to
protect pipes from freezing, or to maintain temperature of process
piping and vessels. An MI resistance heating cable may not be
repairable if damaged.
Typical specifications
| maximum voltage |
600 or 1000 volts |
| current rating |
18 - 450 amperes |
| conductor area |
1.0 - 240 mm² |
| copper sheath area |
5 - 70 mm² effective |
| number of cores |
1,2,3,4,7,12,19 |
| overall diameter |
5 - 26 mm |
| minimum bend radius |
6 x diameter (3 x diameter if bent once only) |
| weight |
100 - 3300 kg/km |
| twists per metre |
0, 20; In many applications NO twist is preferred. |
| finish |
bare copper, standard PVC sheath, low smoke and fume (LSF)
polymer sheath, various Stainless Steels, Inconel, Titanium, and
some super alloys. |
| colour |
natural (Bare Stainless),(bare copper), white, black, red,
orange |
| maximum operating temperature |
| continuous - exposed to touch |
70 °C |
| continuous - not exposed to touch; PVC-sheathed |
90 °C |
| continuous - not exposed to touch; not PVC-sheathed |
250 °C |
| intermittent |
>1000 °C |
| (melting point of copper is 1083
°C) |
|
Advantages
The metal sheath and solid filling of MI cable makes it
mechanically robust and resistant to impact; an MI cable may be
struck repeatedly with a hammer and still provide adequate
insulation resistance for a circuit. Copper sheathing is waterproof
and resistant to ultraviolet light and many corrosive elements. MI
cable is approved by electrical codes for use in areas with
hazardous concentrations of flammable gas in air; an MI cable will
not allow propagation of an explosion inside the copper tube, and
the cable is unlikely to initiate an explosion even during circuit
fault conditions. Metal sheathing will not contribute fuel or
hazardous combustion products to a fire, and cannot propagate a
fire along a
cable tray or within a
building. The cable is inherently
fire-rated without additional
coatings, and will survive designated fire tests representative of
actual fire conditions longer than the enclosing structure.
When used within a tennanted area, carrying electricity supplied
and billed to the landlord, for example for a communal extract
system or antenna booster, it provides a supply cable that cannot
easily be 'tapped' into to obtain free energy.
Although made from solid copper elements, the finished cable
assembly is still pliable due to the
malleability of copper. The cable can be bent
to follow shapes of buildings or bent around obstacles, allowing
for a neat appearance when exposed.
Since the inorganic insulation does not degrade with (moderate)
heating, the finished cable assembly can be allowed to rise to
higher temperatures than plastic-insulated cables; the limits to
temperature rise may be only due to possible contact of the sheath
with people or structures. This may also allow a smaller
cross-section cable to be used in particular applications.
Due to
oxidation, the copper cladding
darkens with age and MICC is therefore often used in historic
buildings such as
castles where it blends in
with stonework.
Disadvantages
- The termination points: While the length of the MI
cable is very tough, at some point, each run of cabling terminates
at a splice or within electrical equipment. These terminations are
vulnerable to fire, moisture, or mechanical impact.
- Vibration: MICC is not suitable for use where it will
be subject to vibration or flexing, for
example connection to heavy or movable machinery. Vibration will
crack the cladding and cores, leading to failure.
- Labour Cost: During installation MI cable must not be
bent repeatedly as this will cause work
hardening and cracks in the cladding and cores. A minimum
bend radius must be observed and the
cable must be supported at regular intervals. The magnesium oxide
insulation is hygroscopic so MICC cable
must be protected from moisture until it has been terminated.
Termination requires stripping back the copper cladding and
attaching a compression gland fitting. Individual conductors are
insulated with plastic sleeves. A sealing tape, insulating putty or
an epoxy resin is then poured into the
compression gland fitting to provide a watertight seal. If a
termination is faulty due to workmanship or damage then the
magnesium oxide will absorb moisture and lose its insulating
properties. Depending on the size and number of conductors, a
single termination can take between 1 to 2 hours of labour.
Installation of a 3-conductor MI cable (size No. 10 AWG (about 5
square mm)) takes about 65% more time than installation of a
PVC-sheathed armored cable of the same conductor size. Installation
of MICC is therefore a costly task. Certain PTFE, silicone or other
polymer-insulated cables have been
substituted in applications which require similar properties in
terms of flame spread, which use less
labour to terminate. MICC is still used in applications which are
particularly suited to its combination of properties.
- Voltage rating: MI cable is only manufactured with
ratings up to 1000 volts.
- Moisture absorption: The magnesium oxide insultion has
a high affinity for moisture. Moisture introduced into the cable
can cause electrical leakage from the internal conductors to the
metal sheath. Moisture absorbed at a cut end of the cable may be
driven off by heating the cable.
- Corrosion: The copper sheath material is resistant to
most chemicals but can be severely damaged by ammonia-bearing
compounds and urine. A pinhole in the copper
sheathing will allow moisture into the insulation, and eventual
failure of the circuit. A PVC over jacket or sheaths of other
metals may be required where such chemical damage is expected. When
MI cable is embedded in concrete as snow melting cable it is
subject to physical damage by concrete workers working the concrete
into the pour. If the 3-5mil coating is damaged pin holes in the
copper jacket develop causing premature failure of the snow melting
system.
- Repair: If the MI cable jacket has been damaged the
magnesium oxide will wick moisture into the cable and it will lose
its insulating properties causing high resistance shorts to the
grounding jacket (Copper Jacket). It is often necessary to remove
2’ to 10’ of the MI Cable and splice in a new section to accomplish
the repair. Depending on the size and number of conductors, a
single termination can take between 1 to 2 hours of labor.
Alternatives
Circuit integrity for conventional
plastic-insulated cables requires additional measures to obtain a
fire-resistance rating or to
lower the
flammability and
smoke contributions to a minimum degree acceptable for
certain types of construction. Sprayed-on coatings or flexible
wraps cover the plastic insulation to protect it from flame and
reduce its flame spreading ability. However, since these coatings
reduce the heat dissipation of the cables, often they must be rated
for less current after application of fire-resistant coatings. This
is called ampacity derating. It can be tested through the use of
IEEE 848 Standard Procedure for the Determination
of the Ampacity Derating of Fire-Protected Cables. The
following materials have been used on their own and/or in
combination with one another for
fireproofing electrical circuits:
So far as
building code and
fire code compliance are concerned, what matters
is
Listing and
approval use and compliance in order to be able to demonstrate
empirically, that the field-installed configuration is capable of
achieving a fire-resistance rating. Whether one uses MI Cable or
ordinary cables that have been externally fireproofed is a matter
of choice, which is largely dictated by economics.
See also
References
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